Measurement instruments that measure, record, process a signal, and display the results of the processing are known to the art. For example, a digital oscilloscope measures the amplitude of a signal as a function of time and displays a portion of the observed signal as a graph of signal amplitude as a function of time. Modern digital oscilloscopes can measure a signal at a rate of close to 100 Gigasamples/sec in each of a plurality of measurement channels. The signal is typically digitized using a bank of sample and hold circuits that sample the signal in successive time slots. Each sample and hold circuit feeds a high speed analog-to-digital converter (ADC) that stores its output in a high speed memory bank that is assigned to that ADC.
Only a small fraction of this recorded data can be displayed at a time, and hence, some form of trigger is needed to define the beginning of the measurements of interest that are to be displayed. In many cases, the user is trying to visualize the signal in regions that satisfy some particular criterion. The criteria of interest can be complex patterns. For example, the user may wish to examine the signal in a region that follows three pulses having specified pulse widths. Hence, the trigger must be able to detect the occurrence of the three pulses.
Unfortunately, prior art triggering schemes that are capable of detecting such complex patterns cannot operate at anything close to the sample rates discussed above, and hence, a two-step triggering system is often employed. To provide a complex trigger, a first trigger that can operate in real time (RTT) is used to control the data that is stored in memory. The RTT is typically implemented in special high-speed hardware that operates on the input signal. This type of trigger is limited to simple patterns such as detecting a rising or falling edge in the input signal. In practice, the digital oscilloscope takes samples continuously and stores these in a circular memory buffer. When a trigger pattern is detected, the digital oscilloscope continues to acquire data for some predetermined period of time that is limited by the buffer size. Data acquisition is then halted to provide time for a slower pattern recognition program that can detect the more complex pattern of interest to examine the stored signal values for the more complex trigger pattern of interest. This type of program is often referred to as a post acquisition trigger (PAT). If the pattern is found, the data from the trigger point onward is then displayed for the user. If the pattern is not found, data acquisition is recommenced and the processes are repeated. During the time in which the PAT is operating, the digital oscilloscope is not acquiring new data, and hence, the digital oscilloscope is “blind”. The blind time is typically a large fraction of the total operating time, and hence, signals of interest can be lost. Hence, it would be advantageous to provide a PAT that operates at higher speeds. Ideally, the PAT would operate in real time, and hence, avoid the two trigger scheme discussed above.